panic: avoid the extra noise dmesg

When kernel panic happens, it will first print the panic call stack,
then the ending msg like:

[ 35.743249] ---[ end Kernel panic - not syncing: Fatal exception
[ 35.749975] ------------[ cut here ]------------

The above message are very useful for debugging.

But if system is configured to not reboot on panic, say the
"panic_timeout" parameter equals 0, it will likely print out many noisy
message like WARN() call stack for each and every CPU except the panic
one, messages like below:

WARNING: CPU: 1 PID: 280 at kernel/sched/core.c:1198 set_task_cpu+0x183/0x190
Call Trace:
<IRQ>
try_to_wake_up
default_wake_function
autoremove_wake_function
__wake_up_common
__wake_up_common_lock
__wake_up
wake_up_klogd_work_func
irq_work_run_list
irq_work_tick
update_process_times
tick_sched_timer
__hrtimer_run_queues
hrtimer_interrupt
smp_apic_timer_interrupt
apic_timer_interrupt

For people working in console mode, the screen will first show the panic
call stack, but immediately overridden by these noisy extra messages,
which makes debugging much more difficult, as the original context gets
lost on screen.

Also these noisy messages will confuse some users, as I have seen many bug
reporters posted the noisy message into bugzilla, instead of the real
panic call stack and context.

Adding a flag "suppress_printk" which gets set in panic() to avoid those
noisy messages, without changing current kernel behavior that both panic
blinking and sysrq magic key can work as is, suggested by Petr Mladek.

To verify this, make sure kernel is not configured to reboot on panic and
in console
# echo c > /proc/sysrq-trigger
to see if console only prints out the panic call stack.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Feng Tang <[e-mail]>
Suggested-by: Petr Mladek <[e-mail]>
Reviewed-by: Petr Mladek <[e-mail]>
Acked-by: Steven Rostedt (VMware) <[e-mail]>
Acked-by: Sergey Senozhatsky <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Jiri Slaby <[e-mail]>
Cc: Sasha Levin <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/tty/sysrq.c

···
 {
 	struct sysrq_key_op *op_p;
 	int orig_log_level;
+	int orig_suppress_printk;
 	int i;
+
+	orig_suppress_printk = suppress_printk;
+	suppress_printk = 0;
 
 	rcu_sysrq_start();
 	rcu_read_lock();
···
 	}
 	rcu_read_unlock();
 	rcu_sysrq_end();
+
+	suppress_printk = orig_suppress_printk;
 }
 
 void handle_sysrq(int key)

include/linux/printk.h

···
 extern char devkmsg_log_str[];
 struct ctl_table;
 
+extern int suppress_printk;
+
 struct va_format {
 	const char *fmt;
 	va_list *va;

kernel/panic.c

···
 	disabled_wait();
 #endif
 	pr_emerg("---[ end Kernel panic - not syncing: %s ]---\n", buf);
+
+	/* Do not scroll important messages printed above */
+	suppress_printk = 1;
 	local_irq_enable();
 	for (i = 0; ; i += PANIC_TIMER_STEP) {
 		touch_softlockup_watchdog();

kernel/printk/printk.c

···
 struct console *console_drivers;
 EXPORT_SYMBOL_GPL(console_drivers);
 
+/*
+ * System may need to suppress printk message under certain
+ * circumstances, like after kernel panic happens.
+ */
+int __read_mostly suppress_printk;
+
 #ifdef CONFIG_LOCKDEP
 static struct lockdep_map console_lock_dep_map = {
 	.name = "console_lock"
···
 	bool in_sched = false, pending_output;
 	unsigned long flags;
 	u64 curr_log_seq;
+
+	/* Suppress unimportant messages after panic happens */
+	if (unlikely(suppress_printk))
+		return 0;
 
 	if (level == LOGLEVEL_SCHED) {
 		level = LOGLEVEL_DEFAULT;